Golf balls were originally made with smooth outer surfaces. In the late nineteenth century, players observed that the guttie golf balls traveled further as they got older and more gouged up. The players then began to roughen the surface of new golf balls with a hammer to increase flight distance. Manufacturers soon caught on and began molding non-smooth outer surfaces on golf balls.
By the mid 1900's, almost every golf ball being made had 336 dimples arranged in an octahedral pattern. Generally, these balls had about 60 percent of their outer surface covered by dimples. Over time, improvements in ball performance were developed by utilizing different dimple patterns. In 1983, for instance, Titleist introduced the TITLEIST 384, which had 384 dimples that were arranged in an icosahedral pattern. About 76 percent of its outer surface was covered with dimples. Today's dimpled golf balls travel nearly two times farther than a similar ball without dimples.
The dimples on a golf ball are important in reducing drag and increasing lift. Drag is the air resistance that acts on the golf ball in the opposite direction from the ball flight direction. As the ball travels through the air, the air surrounding the ball has different velocities and, thus, different pressures. The air exerts maximum pressure at the stagnation point on the front of the ball. The air then flows over the sides of the ball and has increased velocity and reduced pressure. At some point it separates from the surface of the ball, leaving a large turbulent flow area called the wake that has low pressure. The difference in the high pressure in front of the ball and the low pressure behind the ball slows the ball down. This is the primary source of drag for a golf ball.
The dimples on the ball create a turbulent boundary layer around the ball, i.e., the air in a thin layer adjacent to the ball flows in a turbulent manner. The turbulence energizes the boundary layer and helps it stay attached further around the ball to reduce the area of the wake. This greatly increases the pressure behind the ball and substantially reduces the drag.
Lift is the upward force on the ball that is created from a difference in pressure on the top of the ball to the bottom of the ball. The difference in pressure is created by a warpage in the air flow resulting from the ball's back spin. Due to the back spin, the top of the ball moves with the air flow, which delays the separation to a point further aft. Conversely, the bottom of the ball moves against the air flow, moving the separation point forward. This asymmetrical separation creates an arch in the flow pattern, requiring the air over the top of the ball to move faster, and thus have lower pressure than the air underneath the ball.
Almost every golf ball manufacturer researches dimple patterns in order to increase the distance traveled by a golf ball. A high degree of dimple coverage is beneficial to flight distance, but only if the dimples are of a reasonable size. Dimple coverage gained by filling spaces with tiny dimples is not very effective, since tiny dimples are not good turbulence generators.
In addition to researching dimple pattern and size, golf ball manufacturers also study the effect of dimple shape, volume, and cross-section on overall flight performance of the ball. Conventional dimples are the shape of a section of a sphere. These profiles rely on essentially two independent parameters to fully define the dimple shape: diameter and depth (chordal or surface). Edge angle is often discussed when describing spherical dimple profiles but is not independent of diameter and depth. However, it is more commonly used in place of depth when describing spherical dimple shapes. Spherical dimples have a volume ratio (VR) around 0.5 (see below for definition). For purposes of aerodynamic performance, it is desirable to have additional control of dimple shape by varying edge angle independently from dimple diameter and depth. This has been achieved in a number of ways. Examples include “dual radius,” dimple within a dimple, and catenary dimple profiles. These cross-sections allow for more control over spherical cross-sections and allow one to vary VR to optimize aerodynamic performance. With the exception of catenary profiles, the mathematical descriptions are cumbersome or do not result in smooth continuous dimple profiles.
Several patents relate golf ball manufacturers' attempts to construct improved non-spherical golf ball dimples. U.S. Pat. No. 7,094,162 discloses a golf ball dimple comprising a top truncated cone part and a bottom bowl-shaped part. However, this dimple has a sharp demarcation line between these two portions of the dimples which shows a great distinction between them. U.S. Pat. Nos. 4,560,168, 4,970,747, 5,016,887, and 6,454,668 mention dimples having a frusto-conical or truncated cone portion but do not combine that with a bottom spherical portion.
Thus, there still remains a need to construct dimples with a conical portion having a smooth continuous profile and improved aerodynamic performance.